This invention relates to the generation of charged particles, and more particularly, to the control of electrostatic latent images formed from this charged particle source.
A wide variety of techniques are commonly employed to generate ions in various applications. Conventional techniques include air gap breakdown, corona discharges, spark discharges, and others. The use of air gap breakdown requires close control of gap spacing, and typically results in non-uniform latent charge images. Corona discharges, widely favored in electrostatic copiers, provide limited currents and entail considerable maintenance efforts. Electrical spark discharge methods are unsuitable for applications requiring uniform ion currents. Other methods suffer comparable difficulties.
Apparatus and method for generating ions representing a considerable advance over the above techniques are disclosed in copending application Ser. No. 824,252, filed Aug. 12, 1977. The ion generator of this invention, shown in one embodiment at 10 in FIG. 1, involves the use of two conducting electrodes 12 and 13 separated by a solid insulator 11. When a high frequency electric field is applied between these electrodes by source 14, a pool of negative and positive ions is generated in the areas of proximity of the electrode edges and the dielectric surface. Thus in FIG. 1, an air gap breakdown occurs relative to a region 11-r of dielectric 11, creating an ion pool in hole 13-h, which is formed in electrode 13.
These ions may be used, for example, to create an electrostatic latent image on a dielectric member 15 with a conducting backing layer 16. When a switch 18 is switched to position X and is grounded as shown, the electrode 16 is also at ground potential and little or no electric field is present in the region between the ion generator 10 and the dielectric member 15. However, when switch 18 is switched to position Y, the potential of the source 17 is applied to the electrode 13. This provides an electric field between the ion reservoir 11-r and the backing of dielectric member 15. Ions of a given polarity (in the generator of FIG. 1, negative ions) are extracted from the air gap breakdown region and charge the surface of the dielectric member 15.
One advantageous use of this invention, disclosed in the above application, is the formation of characters and symbols in high speed electrographic printing. Apparatus for the formation of dot matrix characters and symbols on dielectric paper or intermediate dielectric members is shown in FIG. 2. A matrix ion generator 20 includes a dielectric sheet 21 with a set of apertured air gap breakdown electrodes 22-1 through 22-4 on one side and a set of selector bars 23-1 through 23-4 on the other side. A separate selector 23 is provided for each different aperture 24 in each finger electrode 22. Ions can only be extracted from an aperture when both its selector bar is energized with a high voltage alternating potential and its finger electrode is energized with a direct current potential applied between the finger electrode and the counterelectrode of the dielectric surface to be charged. Dot matrix characters may be formed using this apparatus by stringing together a series of electrostatic dot images. This is done by moving the dielectric surface to be charged at a prescribed rate past the matrix ion generator 20, and applying direct current pulses to the finger electrodes 22 at a suitable frequency to create a series of overlapping dots.
It has been discovered, however, that this invention suffers a serious disadvantage when utilized in such a dot matrix embodiment, which is illustrated in FIGS. 2 and 3. At an initial time t.sub.1, a given aperture 24.sub.23 on matrix ion generator 20 is energized by a direct current pulse which creates a negative potential on a finger electrode 22-2, while a high frequency potential is applied to selector bar 23-3. This causes the formation of an electrostatic dot image which is negative in polarity, occupying regions 32 and 33 on dielectric surface 30 with backing electrode 31. At a later time t.sub.2, aperture 24.sub.23 is over regions 33 and 34, selector bar 23-3 is still energized, but as charging is not desired, no negative pulse is applied to finger electrode 22-2. The presence of negative electrostatic image in region 33, however, attracts positive ions from the aperture 24.sub.23, erasing the previously created image in this region.
Accordingly it is a principal object of the invention to provide improved apparatus of the type described above for generating ions. A related object of the invention is the achievement of better control over the charging of dielectric members using such ion generating apparatus.
It is another object of the invention to provide a superior matrix printing apparatus using this ion generating principle. A related object is the avoidance of undesired erasures of electrostatic images.